Conventionally, optical disks or optical cards are known as optical information recording media, on which information can be recorded or reproduced optically. These recording media use a semiconductor laser as a light source, and a great deal of information can be recorded or reproduced by irradiation of light that is finely focused through a lens.
At present, there is much research on the above recording media to increase their recording capacities. For example, a recording medium having a multilayer structure whose recording capacity is doubled by laminating information layers to record/reproduce information signals has been proposed (U.S. Pat. No. 5,726,969). Also, a DVD (digital versatile disc)-ROM disk with two information layers has been put into practical use as a read-only optical disk. Moreover, a multilayer recording medium that can be recorded in the user's environment also has been proposed. Such a recording medium is formed of a phase changeable material, a magnet-optical recording material, a dye material, or the like.
On the other hand, an optical recording medium now in use, which includes a single recording layer, employs two types of format for recording signals: a sector-structure system and a continuous recording system. The former is used mainly to record data information, while the latter is used to record sound information, such as CD-R. In an optical disk having a sector structure, an area for managing information to be recorded and a data area on which information signals are recorded by users are separated.
However, when the recording system using a sector structure is applied to a multilayer recording medium, signals reproduced are distorted because of the recorded state of adjacent layers.
FIGS. 12(a) and 12(c) show a cross section of a recordable two-layer recording medium taken along the track direction, respectively. FIGS. 12(b) and 12(d) show the reproduced signals from the information layer of the recording medium. The two-layer recording medium in FIG. 12(a) includes a first information layer 202, a separating layer 203, a second information layer 204, and a protective plate 205 in this order on a substrate 201.
The first information layer 202 has a sector structure including data portions 208 for recording information signals and sector address portions 209 spaced at predetermined lengths of each data portion 208. The sector address portion 209 is used for information management during recording/reproducing information signals. Similarly, the second information layer 204 includes data portions 212 and sector address portions 213.
In FIG. 12(a), the first information layer 202 is not recorded, while a signal is recorded on the second information layer 204. FIG. 12(b) shows a reproduced signal from the second information layer 204. In this case, since the transmissivity of the first information layer 202 is unchanged, the reproduced signal becomes constant according to a pattern that is recorded on the second information layer 204.
On the other hand, in FIG. 12(c), the first information layer 202 is recorded; FIG. 12(d) shows the reproduced signal in this case. Here, the first information layer 202 has such characteristics that the transmissivity is increased by recording information. As shown in FIG. 12(d), the reproduced signal from the second information layer 202 has a waveform whose amplitude is increased in the area corresponding to the recorded portion of the first information layer 202. This is because the transmissivity of the first information layer 202 is raised by recording a signal. When a signal is reproduced from the second information layer 204, light passes through the second information layer 204 twice, i.e., at the time of being focused and reflected. Therefore, the amplitude fluctuation increases in proportion to the square of a change in transmissivity.
As described above, in an optical recording medium having a sector structure, information signals are recorded on the data portions alone and not on the sector address portions. Thus, in reproducing the information signals, the reproduced signal amplitude and the signal level fluctuate significantly depending on the recorded state of the opposite layer. In particular, when the reproduced signal from the second information layer is demodulated, reproduction errors are caused in the area corresponding to the boundary between the sector address portion and the data portion of the first information layer 202, so that the recorded information cannot be demodulated correctly.
Similarly, since the amount of light that reaches the second information layer 204 varies depending on the recorded state of the first information layer 202, information cannot be recorded correctly during recording.